Multi-carrier modulation/demodulation technology often used in multi-carrier transmission systems includes FFT (Fast Fourier Transform)-based OFDM (Orthogonal Frequency Division Multiplexing) and wavelet-based OFDM. Wired transmission lines include a power line, a telephone line, and the like. Power line communication is an example of multi-carrier communication technology applied to wired communication, as disclosed in U.S. Pat. No. 6,507,622B2 (Japanese Patent Laid-open Publication H11-163807), for instance. The FFT-based OFDM enables multi-carrier communication using FFT, thus widely used in both wired and wireless communications, including wireless LAN and power line communication, for example. Compared with the FFT-based OFDM, the wavelet-based OFDM has resistance to inter-carrier interference and superiority in properties.
Further, the FFT-based OFDM requires GI (guard interval) to prevent loss of orthogonality among sub-carriers, while the wavelet-based OFDM requires no GI, thus capable of improving transmission efficiency. FFT-based OFDM processing is not described here since it is widely known. There are two types of wavelets used in the wavelet-based OFDM: a complex-value type and a real-value type. A representative example of the former type is filtered OFDM and the like; the latter type is CMFB (Cosine Modulated Filter Bank) and the like. The wavelet used in the wavelet-based OFDM in the description below is the real-value type. The wavelet-based OFDM is a transmission system based on digital modulation/demodulation processing using a real coefficient wavelet filter bank. The transmission system is one type of a multi-carrier system, in which the real coefficient filter bank combines a plurality of digital-modulated waves and generates a transmitted signal. PAM (Pulse Amplitude Modulation) is used as a modulation scheme for each carrier. Explained below is data transmission in a digital wavelet modulation transmission system (hereinafter referred to as a DWMC transmission system) described above. FIG. 22 shows an example of wavelet waveforms. As shown in FIG. 22, impulse responses of respective sub-carriers in the wavelet waveforms overlap within the respective sub-carriers when transmitted in the digital wavelet modulation transmission system.
FIG. 23 shows an example of a transmitted waveform on a multi-carrier communication apparatus using the wavelet-based OFDM. As shown in FIG. 23, respective transmission symbols form a time waveform, which is a combination of the impulse responses of the respective sub-carriers.
FIG. 24 shows an example of a transmitted spectrum on the multi-carrier communication apparatus using the wavelet-based OFDM. In the DWMC transmission system, several tens to several hundreds of transmission symbols as shown in FIG. 23 form one transmission frame. FIG. 25 shows an example of a configuration within a transmitted frame on the multi-carrier communication apparatus using the wavelet-based OFDM. The transmitted frame includes symbols for information data transmission and preamble symbols used for carrier detection, synchronization, equalization, and the like.
To describe features of the wired transmission lines, a power line in power line communication is explained below as an example. FIG. 26 shows attenuation characteristics of the power line. FIG. 27 shows group delay characteristics of the power line. As shown in FIGS. 26 and 27, attenuation and group delay of the power line are different in frequency bands. Therefore, some frequency bands are available and other frequency bands are not, for the multi-carrier communication apparatus that uses the transmission line such as the power line.
BPSK (Binary Phase Shift Keying) has been used as primary modulation of lowest spectral efficiency (1 bps/Hz) in the FFT-based OFDM. Sub-carriers having communication quality of below a BPSK threshold are masked and not used, thus causing a problem of deterioration in transfer efficiency.
The communication quality of sub-carriers is indicated by CINR (Carrier to Interference and Noise Ratio), CNR (Carrier to Noise Ratio), and the like. In the wavelet-based OFDM, 2PAM is used as a primary modulation scheme of lowest spectral efficiency (2 bps/Hz). Sub-carriers having CINRs below a 2PAM threshold are masked and not used. When a transmission line environment is poor, many sub-carriers are not used, thus causing a similar problem of deterioration in transfer efficiency. In addition to the problem, the wavelet-based OFDM, which uses 2PAM as primary modulation of the lowest spectral efficiency (2 bps/Hz), has no primary modulation comparable to BPSK in the FTT-based OFDM. Thus, the wavelet-based OFDM has a problem where a proportion of masked and unused sub-carriers is higher than the FTT-based OFDM.